This invention relates to a compression ratio changing device for an internal combustion engine, and more particularly to such a device, which can change the compression ratio of the engine by changing the volume of the combustion chamber within the cylinder at the top dead center of the piston.
A compression ratio-changing device of this kind is known e.g. from Japanese Provisional Patent Publication (Kokai) No. 58-91340, in which, as shown in FIG. 1, an eccentric bearing (rotary eccentric member) C is interposed between a piston pin A connected to a piston, not shown, and an end B2 of a connecting rod B remote from a crankshaft, not shown, whereby the position of the piston relative to the connecting rod B can be changed by rotating the eccentric bearing C, thereby changing the volume of the combustion chamber and hence the compression ratio. The eccentric bearing C is locked to and unlocked from the end B2 of the connecting rod B by means of a locking pin D, which is slidably fitted in a guide hole B1 formed in the end B2 of the connecting rod B, and movable in response to oil pressure to be engaged in and disengaged out of a hole C1 formed in the eccentric bearing C.
According to the above conventional arrangement, even when the locking pin D is acted upon by the oil pressure at an end face thereof close to the crankshaft, the locking pin D remains in contact with the outer peripheral surface of the eccentric bearing C without moving, until the rotational angle of the eccentric bearing C, i.e. the piston pin A becomes 90 degrees, as shown in (a)-(c) of FIG. 2, where the eccentric bearing C assumes rotational angles of 0, 30, 60, and 90 degrees, respectively. That is, the locking pin D is not moved as indicated by the points (a)-(c) on the broken line in FIG. 10. When the rotational angle of the piston pin A becomes 90 degrees, the locking pin D rushes into the hole C1, as shown in (d) of FIG. 2, that is, the position of the locking pin D is abruptly changed from the point (d) to the point (e) shown in FIG. 10. At this time, the locking pin D is acted upon by a great bending stress and a great shearing stress produced by the inner peripheral surface of the hole C1, which is disadvantageous in maintaining the strength and durability of the locking pin D.
If, in order to eliminate the disadvantage, the length along which the locking pin D is to be engaged in the hole C1 is shortened, there is a fear of disengagement of the locking pin D from the hole C1 when the locking pin D is given a strong impact by the inner peripheral surface of the hole C1, whereby the eccentric bearing C cannot be firmly locked by the locking pin D.
Another compression ratio-changing device is known e.g. from the Japanese Utility Model Publication (Kokai) No. 62-6263, in which is formed an oil chamber acting as a buffer for mitigating the impact from the hole C1.
However, also in the latter conventional device, the locking pin is disposed to abruptly rush into the hole C1 formed in the eccentric bearing during rotation of the eccentric bearing, similarly to the former conventional device. Therefore, according to the prior art, it is difficult to prevent the looking pin from being acted on by the great bending stress and shearing stress, and hence to firmly and smoothly lock the eccentric member, under various operating conditions of the engine. There is the same disadvantage as mentioned above with another type of compression ratio-changing device in which an eccentric piston pin is used as the eccentric member instead of the eccentric bearing, interposed between the piston and an end of the connecting rod remote from the crank shaft.